In a known way, many electrical and optoelectronic components—in particular but not exclusively those based on organic materials—have to be encapsulated in order to ensure the protection of their sensitive components from oxygen and water vapor. This is the case, for example, with organic light-emitting diodes (“OLED”) and organic photovoltaic cells (“OPVs”). If this protection is not correctly ensured, degradation of the devices, which can result in their failure, takes place. For example, in the case of OLED screens, nonemissive black spots appear, which spots are caused by the degradation, induced by water vapor, of the interface between the anode and/or of the cathode and the thin organic layers.
The simplest encapsulation method for devices sensitive to water and oxygen consists in adhesively bonding a glass cover over the top of said devices, under an inert atmosphere, and in sealing this cover with an epoxy adhesive, for example. As a result of the impossibility of guaranteeing a perfect atmosphere inside the cover and as a result of the lateral permeation through the sealing, this technology is not sufficient. This is why a getter material—that is to say a material which absorbs moisture and/or gas—is typically introduced within this cover. The getter can, for example, be a zeolite, a metal or an oxide, such as CaO or BaO. For example, the document U.S. Pat. No. 7,193,364 describes an encapsulation by an adhesively bonded cover in which the getter form a bead surrounding the component to be protected.
However, the use of a rigid cover as encapsulation means exhibits numerous disadvantages: complex to operate industrially, limitation of the processes which can be carried out after the encapsulation (for example, deposition of colored filters), functional incompatibility (for example for flexible devices) or optical incompatibility (emission through the getter). For example, in the context of top emission OLED (TE-OLED) screens, it is not appropriate to use an encapsulation by a cover as the encapsulation has to be the most transparent possible and the least bulky possible. Furthermore, in the context of flexible screens, this encapsulation has itself also to be flexible. This is why use is preferably made of encapsulation barriers comprising thin layers. “Thin layers” is understood here to mean layers with a thickness of less than or equal to 10 μm, more advantageously less than or equal to 5 μm and preferably less than or equal to 1 μm. Furthermore, advantageously, the thickness of a thin encapsulation layer should not be greater than 15% and preferably than 10% of the smallest lateral dimension of a pixel or a subpixel.
The best thin-layer encapsulations, known under the name of “SHB barriers” (SHB standing for Super-High Barriers), consist of “dyads”, that is to say alternations of inorganic layers (typically oxides) and organic layers. The inorganic layers can be obtained by chemical vapor deposition (CVD), physical vapor deposition (PVD) or atomic layer deposition (ALD), whereas the organic layers can be obtained by CVD, PVD, inkjet printing or spin coating.
In these structures, the oxide acts as impermeable barrier to oxygen and to moisture (water vapor). The organic material is used, on the one hand, to decorrelate the defects between two oxide layers (that is to say, to ensure that a defect present in a lower layer does not spread to an upper layer during the deposition of the latter) and, on the other hand, to lengthen the diffusion path between the defects of two successive layers of oxides. The first organic layer is assumed to have a beneficial effect on the density of defects of the successive layers as it also has a planarization function. See, for example, the documents WO 2000/036665 and WO 2011128802.
Similarly, the document EP 2 136 423 describes an organic optoelectronic component provided with a multilayer encapsulation structure comprising two layers based on perfluorohexane (C6F6), between which are arranged a polymer layer and a layer based on SiOx.
Generally, the more the number of dyads is increased, the more the protective quality of the barrier is improved. However, there are disadvantages to increasing the number of dyads present: increased manufacturing time, difficulty in making contacts, loss of yield and filtering of the colors in the case of OLEDs, and the like.
Furthermore, it is impossible to obtain inorganic barrier layers which are truly devoid of defects (inclusive particles, pinholes and sometimes microcracks). Thus, even if the intrinsic properties of the barriers are excellent, at the location of the defects, the rates of penetration of water and oxygen are very markedly accelerated. The organic layers make it possible to delay the failure of the device (increased diffusion path and defects in the first barrier decorrelated with a possible defect in a second barrier); however, the appearance of a local defect of the OLED is accelerated and, in some cases, it is possible to consider that each defect of a barrier layer results, in the end, in complete failure.
The document U.S. Pat. No. 6,198,220 describes an encapsulation structure comprising a dielectric barrier layer protected by a metal layer. The latter is “self-repairing” in the sense that it reacts with water vapor and/or oxygen so that it automatically reblocks any defect of “pinhole” type. This solution is not very suitable for TE-OLEDs and for organic photovoltaic cells which have to be illuminated via their upper face due to the use of metal layers, even if the latter are sufficiently thin to be partially transparent.
Another method of thin-layer encapsulation known from the prior art consists in using a getter in the form of a thin layer sandwiched between two barrier layers, in order to capture the water which has diffused through the defects of the upper barrier. See, for example, the paper by Byoung Duk Lee et al., “Effect of transparent film desiccant on the lifetime of top-emitting active matrix organic light emitting diodes”, Applied Physics Letters, 90, 103518 (2007).
In accordance with the prior art, use is made, as getter, either of metals, such as Ca, Sr or Ba, or of oxides or sulfides of these metals: CaO, CaS, SrO, SrS, BaO or BaS. These solutions all exhibit disadvantages:
The metals have the major disadvantage of having an effect on the optical properties of the barrier, which is not acceptable, for example in the context of top emission OLED microdisplays.
The oxides and sulfides have the disadvantage of reacting with the water but not with the oxygen and thus do not make possible complete protection of the device. Furthermore, the sulfides produce, on reacting with the water, hydrogen sulfide, which is a malodorous and highly toxic gas.
The document U.S. 2007/0273280 describes an encapsulation structure comprising an alternation of dyads each formed by an organic buffer layer and an “active” barrier layer made of activated metal oxide or oxynitride (that is to say, comprising oxygen deficiences) capable of absorbing the water via a chemical reaction.
The invention is targeted at overcoming the abovementioned disadvantages of the prior art. More particularly, it is targeted at providing a thin encapsulation barrier (thickness of the order of a few micrometers, preferably 1 μm or less) which exhibits a low rate of failure (advantageously lower than that of the SHB barriers of the prior art), is transparent (transmittance of greater than or equal to 90%) and provides effective protection just as well against water vapor as oxygen. Advantageously, such an encapsulation barrier should be compatible with the preparation of a flexible device and with the addition of functional layers (for example: optical filters) above the encapsulation, not appreciably disrupt the passage of light and/or be easy to produce in an industrial environment with a high output.
In accordance with the invention, this aim is achieved by the use of a multilayer encapsulation structure comprising at least one “active” layer interposed between nonmetallic inorganic barrier layers which are impermeable to oxygen and to water vapor. The barrier layers are made of a stoichiometric metal oxide, stoichiometric silicon oxide (SiO2) or a silicon oxynitride (SiNxOy) by atomic layer deposition (ALD). This technique makes it possible to obtain dense stoichiometric layers with only a few defects of “pinhole” type. The active layers consist of (or, in any case, contain) at least one nonstoichiometric oxide exhibiting a shortage of oxygen, for example SiOx with x less than 2 and typically x close to 1 (for example 0.8≤x≤1.2). These oxides capture the oxygen or the water vapor which has crossed an upper barrier layer, typically because of the presence of point defects or pinholes, and react with them, forming stoichiometric oxides, the barrier quality of which makes it possible to compensate, at least in part, for the failure of the upper barrier layer. Furthermore, the reaction of a nonstoichiometric oxide exhibiting an oxygen deficiency with water vapor, H2O, produces gaseous molecular hydrogen H2 which creates a local excess pressure. It is assumed that this excess pressure can have the effect of causing the molecular hydrogen to diffuse through the defect or defects of the barrier layer which covers the active layer while entraining with it the water vapor and the oxygen which have been able to enter therein.